Optical device, method of producing the same, optical pickup, and optical information processing device

ABSTRACT

An optical device having a sub-wavelength grating formed in a specified region is disclosed that is able to prevent wave front degradation accompanying a phase difference of a polarized light beam passing through the optical device. The optical device includes a circular-belt-like region where the sub-wavelength diffraction grating is formed, and a center portion where the sub-wavelength diffraction grating is not formed. A vertically polarized light beam used for operations on a blue-light optical recording medium A has a phase difference in the sub-wavelength diffraction grating to be an integral multiple of 2π and hence is transmitted through the sub-wavelength diffraction grating. A horizontally polarized light beam used for operations on a blue-light optical recording medium is diffracted by the sub-wavelength diffraction grating. The light path length L 1  of the light beam passing through the circular-belt-like region is the same as that of the light beam passing through the center portion without the sub-wavelength grating.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical device used in an opticalpickup for recording, reproducing, or erasing information in an opticalrecording medium, a method of producing the optical device, an opticalproduct, an optical pickup and an optical information processing device.

2. Description of the Related Art

In the related art, an optical pickup in the present technical field isconstituted by assembling many optical parts such as lenses, prisms,wave plates, polarized-light optical devices, and so on. In recent yearsand continuing, it is required that an optical pickup be able to supportnot only conventional CDs or DVDs, but also new optical recording mediaconforming with plural new and old standards, such as large capacityblue-light optical recording media. To meet this requirement, the numberof the optical parts may further increase. On the other hand, the sizeof the optical pick is limited, and cannot be made larger than thislimit. In other words, it is necessary to include DVD, or blue lightfunctions into a space having a CD size. Furthermore, it is required tomake the optical pickup more compact, when, for example, the opticalpickup is installed in a notebook personal computer.

As for the optical parts, such as a prism of a polarized beam splitter,there exist problems of many processing steps and a large space occupiedin the arrangement. Specifically, the prism of the polarized beamsplitter includes two right-angle glass prisms whose shapes areprocessed at high precision, and after a wavelength selection film isformed on the slope of one prism, the one prism is combined with theslope of the other prism, forming a cubic structure. Due to thisstructure, complicated processing steps are required compared to plateelements. In addition, an optical part having such a cubic structureoccupies considerable space inside the optical pickup.

As for materials of optical parts, in the related art, a ¼ wave plate ismade from a quartz crystal, which is an optical crystal, or from aliquid crystal. However, when using the quartz crystal, because it isnecessary to process the optical crystal with the primary axis of thecrystal being in a specified direction, and direction precision is alsorequired when assembling the optical pickup, there is a negative effecton cost reduction. When using the liquid crystal, the liquid crystal hasto be sealed by using two glass substrates, and this also causesincreased cost.

Therefore, it is desired that by integrating plural optical parts intoan optical device, an optical device can be made compact and beassembled simply, and the cost of the optical device can be furtherreduced without using an evaporation film or an optical crystal, whichare fabricated in each conventional optical part.

Along with recent progress in processing techniques, it becomes possibleto fabricate a grating structure having a pitch comparable with thelight wavelength or even shorter. In such a sub-wavelength gratingstructure, although a diffracted wave is not generated, thetransmittance properties strongly depend on the fine structure, and itis possible to control a phase speed (effective refractive index), orproperties of polarized light by controlling the fine structure. Forexample, this technique is described in the following references.

-   1. Hisao Kikuta, Koichi Iwata, “Structural complex refractive index    and its applications to optical devices”, in “Introduction to    Diffraction Optical Devices”, under the editorship of Physical    Society of Applied Physics, Optical Society of Japan, Optical Design    Group, Optronics Co., May 20, 1997, first edition, first copy, pp    158.-   2. Hisao Kikuta, Koichi Iwata, “Optical control with a fine grating    structure comparable to light wavelength”, Optics, Vol 27, pp. 12-17    (1998).-   3. Hisao Kikuta, “Diffraction grating in sub-wavelength region”,    Oplus E, Vol. 21, No. 5 (May, 1999) pp. 543-550.-   4. Japanese Patent Gazette No. 3077156.-   5. Japanese Patent Gazette No. 3382600.

By using the grating structure in the sub-wavelength region, forexample, it is not necessary to perform surface coating of prisms of apolarized beam splitter, and the structure can be made into a plate.Similarly, because the phase speed can be controlled, it is possible toproduce a ¼ wave plate by a grating structure.

FIG. 24 is a schematic view of a polarized light selective diffractiondevice using the sub-wavelength grating in the related art.

In FIG. 24, rectangular gratings having fine pitches are arrangedperiodically at intervals longer than the wavelength of incident light.If a phase difference between the light passing through the rectangulargratings of fine pitches and the light passing through regions otherthan the rectangular gratings is an integral multiple of 2π, theincident light is not diffracted and the incident light beam is totallytransmitted through the diffraction device. While, if the phasedifference is an integral multiple of π, all of the incident light isdiffracted, and there is no light directly passing through thediffraction device. By appropriately selecting the equivalent refractiveindexes and heights of the rectangular gratings of fine pitches, it ispossible to separate the incident polarized light with lightdiffraction. In addition, diffraction direction can be controlled by theshape of the grating of pitches longer than the light wavelength.

FIG. 25 is a schematic view of a wave plate using the sub-wavelengthgrating in the related art.

In FIG. 25, a phase difference is obtainable from anisotropy generatedby the sub-wavelength grating, and since this phase difference can beset to be π or π/2, various kinds of wave plates can be realized.

Further, while the sub-wavelength grating is formed in the whole regionof the optical device in the related art, in an optical pickup of thepresent invention, the sub-wavelength grating is formed only in aspecified limited region to constitute various kinds of opticalelements.

However, when the sub-wavelength grating is formed only in a limitedregion, a light path length difference, and in turn, a phase differenceis generated between the region where the sub-wavelength grating isformed and the region where the sub-wavelength grating is not formed;due to this, a wave front aberration is generated, and this degrades thelight condensing properties of the object lens.

FIG. 26A is schematic view illustrating light paths in thesub-wavelength grating and in the region without the sub-wavelengthgrating in the related art.

As shown in FIG. 26A, the sub-wavelength grating is formed only in aspecified region. If the heights of the sub-wavelength grating and theregion without the sub-wavelength grating are not appropriatelyselected, the light path in the sub-wavelength grating is different fromthe light path in the region without the sub-wavelength grating.

The light path difference Δnd can be expressed byΔnd=(1+ns)*d/2−n1*d

where ns represents a refractive index in a vertical polarizationdirection, and n1 represents a refractive index of the sub-wavelengthgrating region.

This light path difference Δnd generates a phase difference between thesub-wavelength grating region and the region without the sub-wavelengthgrating.

FIG. 26B illustrates an aberration caused by the phase difference in therelated art.

As illustrated in FIG. 26B, due to the light path difference between thesub-wavelength grating region and the region without the sub-wavelengthgrating, and in turn the phase difference between the sub-wavelengthgrating region and the region without the sub-wavelength grating, a wavefront aberration is generated, and this degrades the light condensingproperties of the object lens.

SUMMARY OF THE INVENTION

It is a general object of the present invention to solve one or moreproblems of the related art.

A specific object of the present invention is to provide an opticaldevice, functioning as an optical part with a sub-wavelength gratingbeing formed in a specified region, able to realize a grating structureso that a transmitting light beam does not suffer from wave frontdegradation accompanying a phase difference, a method of producing theoptical device, an optical product, an optical pickup and an opticalinformation processing device.

According to a first aspect of the present invention, there is providedan optical device comprising a sub-wavelength grating formed in at leastone of a plurality of regions, a-refractive index, a grating height or aduty ratio of said sub-wavelength grating being selected such that alight path of an incident first light beam in the sub-wavelength gratingis the same as a light path of the first light beam in neighboringregions, said first light beam being polarized and having apredetermined polarization direction.

As an embodiment, the sub-wavelength grating may be superposed on aportion of a step-like surface, said surface having a plurality ofsteps, said portion including at least two steps. Further, thesub-wavelength grating may be superposed on a grating having a periodlonger than a wavelength of the incident first light beam. Preferably,the sub-wavelength grating may be formed in a circular-belt-like region,or in an inner portion of the circular-belt-like region to diffract anincident second light beam, said second light beam being polarized andhaving a polarization direction perpendicular to the polarizationdirection of the first light beam. Alternatively, the sub-wavelengthgrating may be formed in a circular-belt-like region and in an innerportion of the circular-belt-like region to enable portions of thesub-wavelength grating in the circular-belt-like region and the innerportion of the circular-belt-like region to respectively diffract anincident second light beam to two different directions.

As an embodiment, the sub-wavelength grating may be formed in aplurality of concentric regions, and an incident second light beam haslight paths of different lengths at different borders between eachadjacent two of the concentric regions. Preferably, the sub-wavelengthgrating may be formed to have a plurality of concentric portions in aninner portion of a circular-belt-like region capable of diffraction.

As an embodiment, the sub-wavelength grating functions as an m/2 (m isan integer) wave plate. Preferably, the sub-wavelength grating is formedin one of two regions obtained by symmetrically dividing a plane, andthe sub-wavelength grating may function as a ½ wave plate relative tothe incident second light beam. Alternatively, the sub-wavelengthgrating may be formed inside a circle including a center portion on aplane, or in a region having a plurality of rectangular slices, and thesub-wavelength grating functions as a ½ wave plate relative to theincident second light beam.

According to a second aspect of the present invention, there is provideda method of producing an optical device having a sub-wavelength gratingformed in at least one of a plurality of regions, wherein a light pathof an incident light beam having a predetermined polarization directionin the sub-wavelength grating is the same as a light path of theincident light beam in neighboring regions. The method comprises thesteps of: fabricating a mold having a fine grating shape on a surfacethereof, said fine grating shape being determined by optical design;arranging a light-transmittive and thermal elastic resin on the surfaceof the mold and heating the resin to a melting temperature; pressing theheated resin on the mold; cooling the resin gradually after the resin isshaped by following the mold for shape transcription; and releasing theresin from the mold.

According to a third aspect of the present invention, there is providedan optical product including an optical device having a sub-wavelengthgrating, wherein said sub-wavelength grating is formed in at least oneof a plurality of regions, and a light path of an incident light beamhaving a predetermined polarization direction in the sub-wavelengthgrating is the same as a light path of the incident light beam inneighboring regions, said optical device being produced by a methodcomprising the steps of: fabricating a mold having a fine grating shapeon a surface thereof, said fine grating shape being determined byoptical design; arranging a light-transmittive and thermal elastic resinon the surface of the mold and heating the resin to a meltingtemperature; pressing the heated resin on the mold; cooling the resingradually after the resin is shaped by following the mold for shapetranscription; and releasing the resin from the mold.

According to a fourth aspect of the present invention, there is providedan optical pickup for recording, reproducing or erasing data in at leasttwo optical recording media having numerical apertures of NA1 and NA2(NA1>NA2), respectively, said optical pickup comprising: an opticaldevice having a sub-wavelength grating, said sub-wavelength gratingbeing formed in a circular-belt-like region, and diffracting an incidentsecond light beam of a polarization direction perpendicular to apredetermined polarization direction of a first light beam, wherein inorder to diffract a light beam which is incident only when recording,reproducing or erasing data in the optical recording medium having thenumerical aperture of NA2, the circular-belt-like region of the opticaldevice is made to have diffraction functions to condense or scatter thelight beam to a position different from a condensing point in an innerportion of the circular-belt-like region.

According to a fifth aspect of the present invention, there is providedan optical pickup for recording, reproducing or erasing data in a firstoptical recording medium which uses light of a wavelength λ1, has anumerical aperture of NA1, and has a substrate of a thickness t1, saidoptical pickup comprising: an optical device having a sub-wavelengthgrating, said sub-wavelength grating being formed in an inner portion ofa circular-belt-like region, and diffracting an incident second lightbeam of a polarization direction perpendicular to a predeterminedpolarization direction of a first light beam, wherein in order todiffract a light beam which is incident only when recording, reproducingor erasing data in a second optical recording medium which uses light ofa wavelength λ2 (λ2≧λ1), has a numerical aperture of NA2 (NA2≧NA1), andhas a substrate of a thickness t2 (t2≧t1), the inner portion of thecircular-belt-like region of the optical device is made to havediffraction functions to add an aberration having an opposite polarityto an aberration occurring when the light beam is condensed.

According to a sixth aspect of the present invention, there is providedan optical pickup for recording, reproducing or erasing data in a firstoptical recording medium which uses light of a wavelength λ1, has anumerical aperture of NA1, and has a substrate of thickness t1, saidoptical pickup comprising: an optical device having a sub-wavelengthgrating, said sub-wavelength grating being formed in a plurality ofconcentric regions, and an incident second light beam having differentlight path lengths at different borders between each adjacent two of theconcentric regions, said second light beam being polarized and having apolarization direction perpendicular to a predetermined polarizationdirection of a first light beam, wherein in order to generate differentlight path lengths of a light beam which is incident only whenrecording, reproducing or erasing data in a second optical recordingmedium which uses light of a wavelength λ2 (λ2≧λ1), has a numericalaperture of NA2 (NA2≧NA1), and has a substrate of a thickness t2(t2≧t1), the concentric region of the optical device is made to havephase-step functions to add an aberration having an opposite polarity toan aberration occurring when the light beam is condensed.

According to a seventh aspect of the present invention, there isprovided an optical pickup for condensing light beams from a two-channelarray light source to record, reproduce or erase data in an opticalrecording medium, said optical pickup comprising: an optical devicehaving a sub-wavelength grating, said sub-wavelength grating beingformed in one of two regions obtained by symmetrically dividing a plane,and functioning as a ½ wave plate relative to an incident second lightbeam having a polarization direction perpendicular to a predeterminedpolarization direction of a first light beam, wherein the two lightbeams output from the two-channel array light source are incident on theoptical recording medium with the polarization directions of the twolight beams being substantially perpendicular to each other.

According to an eighth aspect of the present invention, there isprovided an optical pickup, comprising: an optical device having asub-wavelength grating, said sub-wavelength grating being formed insidea circle including a center portion on a plane, or in a region having aplurality of rectangular slices, and said sub-wavelength gratingfunctioning as a ½ wave plate relative to a second light beam having apolarization direction perpendicular to a polarization direction of afirst light beam; and a polarization filter arranged between the opticaldevice and an object lens, wherein a polarization direction of a lightbeam incident on the optical device is set to be substantiallyperpendicular near an optical axis of the light beam, and a portion of alight beam incident on the object lens is filtered by the polarizationfilter to form a super high resolution beam spot on an optical recordingmedium.

According to a ninth aspect of the present invention, there is providedan optical information processing device for recording, reproducing orerasing data in an optical recording medium, said optical informationprocessing device comprising: a sub-wavelength grating formed in atleast one of a plurality of regions, a refractive index, a gratingheight or a duty ratio of said sub-wavelength grating being selectedsuch that a light path of an incident first light beam having apredetermined polarization direction in the sub-wavelength grating isthe same as a light path of the first incident light beam in neighboringregions.

According to the present invention, in an optical device havingpolarized light diffraction functions realized by a sub-wavelengthgrating, and an optical pickup having such an optical device, a phasedifference does not occur between a region where the sub-wavelengthgrating is formed and a region where the sub-wavelength grating is notformed, and due to this, undesirable wave front degradation does nothappen. Therefore, it is possible to obtain an optical device havingfunctions of aperture limitation for polarized light selection,aberration correction, phase shifter, or functions of a wave plate, andby incorporating these optical devices, it is possible to obtain anoptical pickup of high compatibility, a multi-beam optical pickup, and asuper-high resolution optical pickup; and an optical informationprocessing device using these optical pickups.

In addition, according to the present invention, it is possible toobtain an optical device which can be fabricated without a surfacecoating process, which enables functions of multiple parts to be shared,and enables reduction of the size and the cost.

These and other objects, features, and advantages of the presentinvention will become more apparent from the following detaileddescription of preferred embodiments given with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating an optical pickup according to afirst embodiment of the present invention;

FIG. 2 is schematic view illustrating light paths in the sub-wavelengthgrating and in the region where the sub-wavelength grating is notformed;

FIG. 3A is a perspective view of an aperture limitation elementaccording to the present embodiment;

FIG. 3B is a cross-sectional view of the aperture limitation elementalong the line AA′ in FIG. 3A;

FIG. 4 is a plan view illustrating an example of the sub-wavelengthgrating according to the present embodiment;

FIG. 5 shows the wave front aberration generated when using the objectlens designed for the optical recording medium A for the object lens ofthe optical recording medium B;

FIG. 6 shows a relation between substrate thickness of an opticalrecording medium and divergence of a light beam from a common objectlens;

FIG. 7A is a perspective view of an aberration correction elementaccording to the present embodiment;

FIG. 7B is a cross-sectional view of the aberration correction elementalong the line AA′ in FIG. 7A;

FIG. 8 is a perspective view showing an example of an optical deviceaccording to the present embodiment;

FIG. 9A through 9E are cross-sectional views illustrating a process offabricating a mold according to the present embodiment;

FIG. 10A through 10C are cross-sectional views illustrating a process oftranscribing a grating pattern;

FIG. 11 is a schematic view illustrating an optical pickup according toa second embodiment of the present invention;

FIG. 12A is a perspective view of an aberration correction element usinga polarized light phase shifter according to the present embodiment;

FIG. 12B is a cross-sectional view of the aberration correction elementalong the line AA′ in FIG. 12A;

FIG. 13A shows the wave front aberration to be corrected;

FIG. 13B shows the wave front aberration after correction;

FIG. 14 is a schematic view illustrating an optical pickup according toa third embodiment of the present invention;

FIG. 15 shows calculation results of a relation between an incidenceangle relative to the object lens and the wave front aberration;

FIG. 16A through FIG. 16C illustrate examples of the two-beam light pathcombination element;

FIG. 17A and FIG. 17B illustrate examples of the light beam deflectionelement;

FIG. 18A is a perspective view of a polarization direction switchingelement using functions of a wave plate of a sub-wavelength gratingaccording to the present embodiment;

FIG. 18B is a cross-sectional view of the polarization directionswitching element along the line AA′ in FIG. 18A;

FIG. 19 is a schematic view illustrating an optical pickup according toa fourth embodiment of the present invention;

FIG. 20A is a perspective view of a super high resolution elementfunctioning as a polarization direction switching element according tothe present embodiment;

FIG. 20B is a cross-sectional view of the super high resolution element35 along the line AA′ in FIG. 20A;

FIG. 21 is a perspective view of the super high resolution elementillustrating an example of the light-shielding area;

FIG. 22 is a perspective view of the super high resolution elementillustrating another example of the light-shielding area;

FIG. 23 is a perspective view schematically illustrating an opticalinformation processing device according to a fifth embodiment of thepresent invention;

FIG. 24 is a schematic view of a polarized light selective diffractiondevice using the sub-wavelength grating in the related art;

FIG. 25 is a schematic view of a wave plate using the sub-wavelengthgrating in the related art;

FIG. 26A is schematic view illustrating light paths in thesub-wavelength grating and in the region without the sub-wavelengthgrating in the related art; and

FIG. 26B illustrates an aberration caused by the phase difference in therelated art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Below, preferred embodiments of the present invention are explained withreference to the accompanying drawings.

In the following embodiments, descriptions are made of compatibledevices of an optical pickup for recording, reproducing or erasing datain at least two optical recording media which use light of differentwavelengths, substrates of different thickness, and have differentnumerical apertures; and the sub-wavelength grating functions as apolarized light diffraction device and a polarized light phase shifter.

First Embodiment

FIG. 1 is a schematic view illustrating an optical pickup according to afirst embodiment of the present invention.

The optical pickup illustrated in FIG. 1 is used for recording,reproducing or erasing data in an optical recording medium A and anoptical recording medium B which use light of different wavelengths,substrates of different thicknesses, and have different numericalapertures. Specifically, the optical recording medium A is a blue lightoptical recording medium which uses light of a wavelength equaling 405nm, has a numerical aperture NA 0.85, and has a substrate of thicknessequaling 0.1 mm on the incidence side. The optical recording medium B isa blue light optical recording medium, which uses light of a wavelengthequaling 405 nm, has a numerical aperture NA 0.65, and has a substrateof thickness equaling 0.6 mm on the incidence side.

A principal portion of the optical pickup illustrated in FIG. 1 includesa semiconductor laser 1 emitting a light beam of wavelength of 405 nm, acollimator lens 2, a polarization plane switching element 3, a halfmirror 4, a deflection prism 5, a polarized light selective aperturelimitation element 6, a polarized light selective aberration correctionelement 7, an object lens 8, a detection lens 10, a light beam divider11, and a light receiving element 12.

The object lens 8 is designed such that for a light beam incident on theblue light optical recording medium A 9 a (wavelength: 405 nm, numericalaperture: NA=0.85, substrate thickness on the incidence side: 0.1 mm) inparallel, the wave front aberration is a minimum. A parallel incidencesystem is also referred to as “infinite incidence system”.

Generally, when the numerical aperture NA an object lens is higher, abetter tolerance is required. In the present example, compared to thenumerical aperture NA 0.65, it is relatively difficult to obtain desiredproperties with the numerical aperture NA 0.85; hence, it is preferableto use a non-spherical lens of a numerical aperture NA 0.85 withaberration being corrected.

The optical recording medium A 9 a and the optical recording medium B 9b have substrates of different thicknesses and use light of differentwavelengths. The optical recording medium A 9 a is a blue light opticalrecording medium, which has a substrate of thickness equaling 0.1 mm onthe incidence side. The optical recording medium B 9 b is a blue lightoptical recording medium, which has a substrate of thickness equaling0.6 mm on the incidence side. When recording or reproducing data, one ofthe optical recording media 9 a and 9 b is set in a not-illustratedrotational mechanism and is rotated at high speed.

Below, a description is made of operations of recording or reproducingdata in the optical recording medium 9 a, that is, the blue lightoptical recording medium A related to a wavelength of 405 nm, numericalaperture NA 0.85, and a substrate thickness of 0.1 mm on the incidenceside.

A linearly-polarized and divergent light beam having a wavelength of 405nm is emitted from the semiconductor laser 1. This linearly-polarizedand divergent light beam is converted into a substantially parallel beamin the collimator lens 2. In the polarization plane switching element 3,the polarization direction of the light beam is rotated by 90 degrees ina plane perpendicular to the paper. Assume that the polarizationdirection of the light beam after rotation is perpendicular to thepolarization direction of the incident light beam. The thus obtainedlight beam passes through the half mirror 4, and the light path of thelight beam is deflected by 90 degrees in the deflection prism 5. Then,the light beam passes through non-sensitive regions of the polarizedlight selective aperture limitation element 6 and the polarized lightselective aberration correction element 7, is incident on the objectlens 8, and is condensed on the optical recording medium 9 a to be afine spot. With this spot, data recording, reproduction, or erasure inthe optical recording medium 9 a is performed.

The light beam reflected from the optical recording medium 9 a isconverted into substantially a parallel light beam again, and isreflected by the half mirror 4. The reflected light beam is focused tobe a focused light beam by the detection lens 10, divided into pluraldivisional light beams in the light beam divider 11 along differentlight paths, and arrives at the light receiving element 12. The lightreceiving element 12 outputs information signals and servo signals.

Next, a description is made of operations of recording or reproducingdata in the optical recording medium 9 b, that is, the blue lightoptical recording medium B having a wavelength of 405 nm, numericalaperture NA 0.65, and a substrate thickness of 0.6 mm on the incidenceside.

A linearly-polarized and divergent light beam having a wavelength of 405nm is emitted from the semiconductor laser 1. This linearly-polarizedand divergent light beam is converted into a substantially parallel beamin the collimator lens 2. In the polarization plane switching element 3,the polarization direction of the light beam is not rotated and passesthrough with the polarization direction of the light beam being the sameas the polarization direction of the incident light beam. The thusobtained light beam passes through the half mirror 4, and the light pathof the light beam is deflected by 90 degrees in the deflection prism 5.

Then, the numerical aperture NA the light beam is limited to be 0.65 inthe polarized light selective aperture limitation element 6, and isdiffracted in the polarized light selective aberration correctionelement 7. This light beam is incident on the object lens 8 with certaindivergence, and is condensed on the optical recording medium 9 b as afine spot. With this spot, data recording, reproduction, or erasure inthe optical recording medium 9 b is performed.

The light beam reflected from the optical recording medium 9 b isconverted into a substantially parallel light beam again, and isreflected by the half mirror 4. The reflected light beam is focused tobe a focused light beam by the detection lens 10, is divided into pluraldivisional light beams in the light beam divider 11 along differentlight paths, and arrives at the light receiving element 12. The lightreceiving element 12 outputs information signals and servo signals.

The polarization plane switching element 3 may be formed from a TwistedNematic liquid crystal. As is well known, when a voltage is applied onthe Twisted Nematic liquid crystal, the polarization direction of theincident light beam is rotated by 90 degrees, and when the voltage isnot applied, the polarization direction of the incident light beampasses without any change.

Next, a description is made of a relationship between the numericalaperture (NA) and the diameter of the light beam. In the presentembodiment, it is necessary to change the numerical aperture (NA)according to the object light optical recording medium. Specifically,while the blue light optical recording medium A 9 a has a numericalaperture NA 0.85, the blue light optical recording medium A 9 b has anumerical aperture NA 0.65. The numerical aperture NA can be expressedby the following formula (1), $\begin{matrix}{{NA} = \frac{\phi}{2f}} & (1)\end{matrix}$

where, f represents the focal length of the object lens, and φrepresents an effective diameter of the light beam to be focused.

Therefore, in order to change the numerical aperture (NA) correspondingto the object light optical recording medium, it is sufficient toprovide a unit to change the numerical aperture (NA) according to theobject light optical recording medium. For this purpose, thepolarization plane switching element 3 and the polarized light selectiveaperture limitation element 6 are provided. The former is used to changethe polarization direction of the light beam output from the lightsource according to the object light optical recording medium, and thelatter has zero-order or first-order diffraction functions according tothe polarization direction of the incident light beam.

In diffraction of polarized light for aperture limitation, by using thepolarized light selective aperture limitation element having asub-wavelength grating, as illustrated in FIG. 24, if the equivalentrefractive index of a portion of a rectangular grating having finepitches and the height of the grating are appropriately selected, it ispossible to separate the polarized light by light diffraction. Inaddition, the direction of diffraction can be controlled by the shape ofthe grating having pitches longer than the light wavelength of theincident light beam.

Next, a description is made of the conditions under which a phasedifference does not occur between a light beam having a predeterminedpolarization direction through a region where the sub-wavelength gratingis formed and the light beam through a region where the sub-wavelengthgrating is not formed in an optical device.

The sub-wavelength grating has a complex refractive index which dependson whether the polarization direction of the incident light is parallelor perpendicular to a channel direction of the grating.

The refractive index of a portion having a periodic structure,specifically, an np component in a horizontal polarization direction ofthe incident light beam and an ns component in a perpendicularpolarization direction of the incident light beam, can be expressed bythe following formulae (2) and (3). $\begin{matrix}{{np} = \sqrt{{t({n1})}^{2} + {( {1 - t} )({n2})^{2}}}} & (2) \\{{ns} = \frac{1}{\sqrt{\frac{t}{({n1})^{2}} + \frac{1 - t}{({n2})^{2}}}}} & (3)\end{matrix}$

where, n1, n2 represent the refractive indexes of materials at tops andbottoms in the grating periodic structure, and t represents a dutyratio.

If n2 is set to be 1 (that is, refractive index of air), and the dutyratio t is set to be 0.5, the np component and the ns component can beexpressed by the following formulae (4) and (5). $\begin{matrix}{{np} = \sqrt{\frac{({n1})^{2}}{2} + \frac{1}{2}}} & (4) \\{{ns} = \frac{1}{\sqrt{\frac{1}{2({n1})^{2}} + \frac{1}{2}}}} & (5)\end{matrix}$

For example, for the incident light beam having a perpendicularpolarization direction, in order to obtain a non-sensitive region (thatis, the phase difference δ is an integral multiple of 2π), it issufficient to satisfy the following formula (6). $\begin{matrix}{\delta = {{\frac{2\pi\quad{d1}}{\lambda}( {{ns} - 1} )} = {2m\quad\pi}}} & (6)\end{matrix}$

where, λ indicates the wavelength of the incident light beam, d1represents a height of the grating, and m is an arbitrary integer.

FIG. 2 is schematic view illustrating light paths in the sub-wavelengthgrating and in the region where the sub-wavelength grating is notformed.

As illustrated in FIG. 2, in order that a phase difference does notoccur between a light beam through a region where the sub-wavelengthgrating is formed and the light beam through a region where thesub-wavelength grating is not formed, it is required that the light pathof the incident light beam in the sub-wavelength grating equal the lightpath of the light beam in neighboring regions where the sub-wavelengthgrating is not formed.

Therefore, the average light path length L1 of a light beam through theregion where the sub-wavelength grating is formed can be expressed bythe following formula (7). $\begin{matrix}{{L1} = {( {{ns} + 1} )\frac{d1}{2}}} & (7)\end{matrix}$

It is sufficient to set the path length L1 to be the same as the lightpath length L2 of the light beam through the region where thesub-wavelength grating is not formed.

The light path length L2 is given by the following formula (8),L2=n1 d2  (8)

where, n1 represents the refractive index of the region where thesub-wavelength grating is formed, and d2 corresponds to the height ofthe region where the sub-wavelength grating is not formed.

Next, a description is made of the polarized light selective aperturelimitation element 6, which utilizes the function of polarized lightselective diffraction for aperture limitation.

FIG. 3A is a perspective view of an aperture limitation elementaccording to the present embodiment, and FIG. 3B is a cross-sectionalview of the aperture limitation element along the line AA′ in FIG. 3A.

Here, when recording or reproducing data in the blue-light opticalrecording medium A, a light beam of a vertical polarization direction isincident on and passes through the aperture limitation element 6, andwhen recording or reproducing data in the blue-light optical recordingmedium B, a light beam of a horizontal polarization direction isincident on and passes through the aperture limitation element 6.

As illustrated in FIG. 3A, the optical device includes acircular-belt-like (annular) region 6 b where a sub-wavelengthdiffraction grating is formed, and a center portion 6 a where adiffraction grating is not formed. At the border line between the centerportion 6 a and the circular-belt-like region 6 b, the numericalaperture NA equals 0.65. The circular-belt-like region 6 b is alsocalled an aperture limitation region.

When recording or reproducing data in the blue-light optical recordingmedium A, it is required that the light beam incident on the aperturelimitation element 6 penetrate the circular-belt-like region 6 b aszero-order diffraction light. For this purpose, it is sufficient thatthe phase difference δ generated in the aperture limitation element 6 bean integral multiple of 2π for the vertically polarized incident lightbeam, that is, the following formula (9) is satisfied, $\begin{matrix}{\delta = {{\frac{2\pi\quad{d1}}{\lambda}( {{ns} - 1} )} = {2m\quad\pi}}} & (9)\end{matrix}$

where d1 represents channel depth of the diffraction grating of thecircular-belt-like region 6 b, λ indicates the wavelength of theincident light beam, d1 represents the height of the grating, and m isan arbitrary integer.

When recording or reproducing data in the blue-light optical recordingmedium B, the light beam incident on the aperture limitation element 6is horizontally polarized and is diffracted.

A diffraction efficiency η1 of a thin phase diffraction grating having across section of a rectangular structure, as illustrated in FIG. 3B, canbe expressed by the following formula (10), $\begin{matrix}{{\eta 1} = {\frac{4}{\pi^{2}}{\sin^{2}( {\frac{\Delta\quad{nd1}}{\lambda}\pi} )}}} & (10)\end{matrix}$

where Δn represents a complex diffraction index induced by a super finediffraction grating.

In the present embodiment, when recording or reproducing data in theblue-light optical recording medium A, it is required that the lightbeam incident on the aperture limitation element 6 pass through anon-sensitive region. For this purpose, the path lengths of the lightbeam parts passing through the circular-belt-like region 6 b and thecenter portion 6 a should be the same. If a difference of the pathlengths exists, the light beam passing through may experience a phasedifference and generate an aberration. To avoid this problem, thediffraction grating of the circular-belt-like region 6 b having pitcheslonger than the light wavelength has a cross section formed bydepressions and projections, and the average light path length L1 of thedepressions and projections can be expressed by the following formula(11). $\begin{matrix}{{L1} = {( {\frac{1 + {ns}}{2} + 1} )\frac{d1}{2}}} & (11)\end{matrix}$

It is sufficient to set the path length L1 to be the same as the lightpath length L2 of the light beam part through the center portion 6 awhere no diffraction grating is formed. The light path length L2 isgiven by the following formula (12).L2=n1 d2  (12)

The light beam incident on the center portion 6 a and thecircular-belt-like region 6 b is diffracted in the forward path, and iscondensed on the optical recording medium 9 a. After that, the lightbeam is diffracted again at a position at which the diffraction gratingis axially symmetric relative to the optical axis of the diffractiongrating, that is, the position of rotational symmetry.

When the grating pattern of the diffraction grating is axially symmetricrelative to the optical axis of the diffraction grating, the lightdiffracted of positive orders in the forward path is diffracted ofnegative orders in the returning path, and becomes zero-order light tobe superposed on the light moving straightforward. If the superposedlight moving straightforward enters the light receiving element, thelight, as a Flare component, degrades the signals output from the lightreceiving element.

FIG. 4 is a plan view illustrating an example of the sub-wavelengthgrating according to the present embodiment.

To solve the above mentioned problem, as illustrated in FIG. 4, in thecircular-belt-like region 6 b, the planar grating pattern of thediffraction grating 6 b is divided into two sections 6 e and 6 f, andthe two grating patterns 6 e and 6 f are perpendicular to each other soas to eliminate the rotational symmetry. With such a structure, thelight diffracted of positive (or negative) orders in the forward path,and the light diffracted of negative (or positive) orders in thereturning path do not superpose with each other, and this reduces theflare component.

FIG. 5 shows the wave front aberration generated when using the objectlens designed for the optical recording medium A 9 a as the object lensof the optical recording medium B 9 b, where the abscissa indicates thediameter of the incidence aperture, and the ordinate indicates the wavefront aberration.

Specifically, the object lens for the blue-light optical recordingmedium A 9 a is of NA=0.85, and is designed to obtain good aberrationproperties on the blue-light optical recording medium A 9 a, which useslight of wavelength 405 nm, and has a substrate of thickness 0.1 mm onthe incidence side.

When this object lens is used for the blue-light optical recordingmedium B 9 b using light of wavelength 405 nm and having a substrate ofthickness 0.6 mm on the incidence side, with the numerical aperture NAof the object lens being limited to be 0.65, a wave front aberration isgenerated as illustrated in FIG. 5.

In FIG. 5, a cross section of a two dimensional phase differencedistribution is presented, but in practice, the phase differencedistribution is a three-dimensional rotational-symmetric one with thevertical axis (NA=0) being the symmetric axis.

In order to correct such an aberration, similar to the aperturelimitation technique as described above, the function of polarized lightselective diffraction can be used.

FIG. 6 shows a relation between substrate thickness of an opticalrecording medium and divergence of a light beam from a common objectlens, where the abscissa indicates the substrate thickness of theoptical recording medium, and the ordinate indicates a magnification ofthe object lens in usage state, the magnification being a function ofthe divergence of the light beam incident on the object lens.

Because the light beam being emitted from the object lens to thesubstrate side is always a focused beam, here a sign “+” is used toindicate incidence of a focused beam into the object lens, and a sign“−” is used to indicate incidence of a divergent beam into the objectlens. Particularly, a magnification equaling 0 indicates that a parallelbeam is incident into the object lens.

In FIG. 6, each point in the graph corresponds to a magnificationresulting in a minimum of the wave front aberration at a given substratethickness of the optical recording medium. For example, as is wellknown, if parallel incidence is optimum at a given substrate thicknessof the optical recording medium, the thicker the substrate, the moredivergent (that is, farther in the—direction) the incident light beam isrequired to be;, the thinner the substrate, the more focused (that is,farther in the +direction) the incident light beam is required to be;,and by selecting the incident light beam in this way, the aberration canbe reduced. Specifically, in the present embodiment, the object lens isoptimized for the blue-light optical recording medium A having asubstrate thickness of 0.1 mm, and when the object lens is used forlight condensing to the blue-light optical recording medium B 9 b havinga substrate thickness of 0.6 mm, a divergent incident light beam can beused to reduce the aberration.

Next, a description is made of an aberration correction elementutilizing the function of polarized light selective diffraction.

FIG. 7A is a perspective view of an aberration correction elementaccording to the present embodiment, and FIG. 7B is a cross-sectionalview of the aberration correction element along the line AA′ in FIG. 7A.

Here, when recording or reproducing data in the blue-light opticalrecording medium A, a light beam of a vertical polarization direction isincident on and passes through the aberration correction element, andwhen recording or reproducing data in the blue-light optical recordingmedium B, a light beam of a horizontal polarization direction isincident on and passes through the aberration correction element.

As illustrated in FIG. 7A, the optical device includes a center portion7 a of a circular-belt-like region, namely, a region on an inner side ofthe circular-belt-like region, where a diffraction grating is formed,and a peripheral region 7 b, where a diffraction grating is not formed.At the border line between the center portion 7 a and the peripheralregion 7 b, the numerical aperture NA is equal to or greater than 0.65.

When recording or reproducing data in the blue-light optical recordingmedium A, it is required that the light beam passing through theaberration correction element penetrate the center portion 7 a aszero-order diffraction light. For this purpose, it is sufficient thatthe phase difference δ generated in the aberration correction element bean integral multiple of 2π for the vertically polarized incident lightbeam.

When recording or reproducing data in the blue-light optical recordingmedium B, an incident light beam is used which has a polarizationdirection perpendicular to the optical recording medium A. When such alight beam is incident, a phase difference is generated and the incidentlight beam is diffracted. The planar pattern of the diffraction regionmay be the well-known zone-plate.

In the present embodiment, the aforesaid aperture limitation element andthe aberration correction element may be integrated together. Forexample, if the aperture limitation function and the aberrationcorrection function are provided on two sides of a plate-shaped opticalelement, respectively, it is possible to reduce the number of the parts.

FIG. 8 is a perspective view showing an example of an optical deviceaccording to the present embodiment.

As illustrated in FIG. 8, in the optical device, a circular-belt-likeregion 6 b having the aperture limitation function and a center portion7 a having the aberration correction function are provided on the sameplane.

Next, descriptions are made of methods of producing the above mentionedoptical devices according to the present invention.

The method of producing the optical device mentioned above according tothe present invention involves using a light-transmittive and thermalelastic resin as the material of the diffraction grating, andtranscribing a fine shape formed on a mold for processing the resin.Therefore, it is not necessary to execute time-consuming processes, suchas evaporation, as in the related art, and thereby enabling massproduction.

FIG. 9A through 9E are cross-sectional views illustrating a process offabricating a mold according to the present embodiment.

In the step shown in FIG. 9A, a mold 40 for molding resin is exposedwith an electron beam.

In the step shown in FIG. 9B, the mold 40 is then developed.

In the step shown in FIG. 9C, reactive ion etching or another isexecuted on the mold 40.

In the step shown in FIG. 9D, due to the reactive ion etching, the moldsubstrate is dug, forming a prototype mold.

In the step shown in FIG. 9E, a separation material 41 is applied on theprototype mold for separating the mold 40 easily in the followingtranscription step.

FIG. 10A through 10C are cross-sectional views illustrating a process oftranscribing a grating pattern.

In the step shown in FIG. 10A, a light-transmittive and thermal elasticresin 46 is applied on a substrate 45.

The resin 46 is pressed against the mold 40, and the surface of theresin 46 is heated to a melting temperature that is slightly higher thana resin Tg temperature, for example, the glass transition temperature ofa resin of a high transmittance, such as CYTOP, is 108° C., and is 105to 120° C. for PMMA.

In the step shown in FIG. 10B, the heated resin 46 is pressed on themold 40 and pressure is applied. Afterward, the resin 46 is graduallycooled after the resin 46 is shaped based on the mold 40 for shapetranscription.

In the step shown in FIG. 10C, the resin 46 is released from the mold40.

Following the steps in FIGS. 9A through 9E and FIGS. 10A through 10C,the diffraction grating can be produced.

In the present embodiment, along with transcription of the diffractiongrating to the optical device, another grating may be formed on theother surface of the optical device. In doing so, in an optical productor an optical pickup, which includes the above optical device,specifically, includes at least one of a wave plate, polarized-lightoptical element, aperture limitation element, aberration correctionelement, and a phase shifter, the number of parts can be reduced, andthe fabrication cost and size of the device can be reduced.

According to the present embodiment, it is possible to realize acompatible optical pickup for recording, reproducing or erasing data inboth a blue-light optical recording medium A, which uses light ofwavelength 405 nm, has a numerical aperture NA 0.85, and a substratethickness of 0.1 mm on the incidence side; and a blue-light opticalrecording medium B, which uses light of wavelength 405 nm, has anumerical aperture NA 0.65, and a substrate thickness of 0.6 mm on theincidence side.

Second Embodiment

In the present embodiment, an optical pickup is able to record,reproduce or erase data in a blue-light optical recording medium A,which uses light of wavelength 405 nm, having a numerical aperture NA0.85, and having a substrate thickness of 0.1 mm on the incidence side;and is able to record, reproduce or erase data in a blue-light opticalrecording medium B, which uses light of wavelength 405 nm, has anumerical aperture NA 0.65, and a substrate thickness of 0.6 mm on theincidence side. Furthermore, the optical pickup is able to record,reproduce or erase data in a DVD optical recording medium C, which useslight of wavelength 660 nm, having a numerical aperture NA 0.65, andhaving a substrate thickness of 0.6 mm on the incidence side; and a CDoptical recording medium D, which uses light of wavelength 785 nm, has anumerical aperture NA 0.50, and a substrate thickness of 1.2 mm on theincidence side.

FIG. 11 is a schematic view illustrating an optical pickup according toa second embodiment of the present invention, which is configured torecord, reproduce or erase data in a blue-light optical recording mediumA, which uses light of wavelength 405 nm, having a numerical aperture NA0.85, and having a substrate thickness of 0.1 mm on the incidence side;and a DVD optical recording medium C, which uses light of wavelength 660nm, having a numerical aperture NA 0.65, and having a substratethickness of 0.6 mm on the incidence side.

A principal portion of the optical pickup illustrated in FIG. 11includes a blue light optical system and a DVD optical system. The bluelight optical system is for processing a light beam of wavelength 405nm, including a blue-light semiconductor laser 1 emitting a light beamof wavelength 405 nm, a collimator lens 2, a half mirror 4, a dichroicprism 24, a deflection prism 5, a polarized light selective aperturelimitation element 6, a polarized light selective aberration correctionelement 7, an object lens 8, a detection lens 10, a light beam divider11, and a light receiving element 12.

The DVD optical system is for processing a light beam of wavelength 660nm, including a hologram unit 21, a collimator lens 22, a dichroic prism24, the deflection prism 5, the polarized light selective aperturelimitation element 6, the polarized light selective aberrationcorrection element 7, and the object lens 8.

The object lens 8 is designed such that for a light beam incident on theblue light optical recording medium A (wavelength: 405 nm, numericalaperture: NA=0.85, substrate thickness on the incidence side: 0.1 mm) inparallel, the wave front aberration is minimum. Such a parallelincidence system is also referred to as “infinite incidence system”.

Generally, when the numerical aperture NA of an object lens is higher, abetter tolerance is required. In the present example, compared to thenumerical aperture NA=0.65, it is relatively difficult to obtain desiredproperties with the numerical aperture NA=0.85; hence, it is preferableto use a non-spherical lens of a numerical aperture NA=0.85 withaberration being corrected.

In addition, the polarization direction of the light from the blue-lightsemiconductor laser 1 is perpendicular to the polarization direction ofthe light from the hologram unit 21 for DVD use, and the systems arearranged so that the light beam to be focused on the DVD is diffractedby the polarized light selective aperture limitation element 6 and thepolarized light selective aberration correction element 7, and the bluelight beam to be focused on the blue light optical recording medium Apasses through non-sensitive regions of the polarized light selectiveaperture limitation element 6 and the polarized light selectiveaberration correction element 7.

Below, a description is made of operations of recording or reproducingdata in the optical recording medium 9 a, that is, the blue lightoptical recording medium A related to a wavelength of 405 nm, numericalaperture of NA=0.85, and a substrate thickness of 0.1 mm on theincidence side.

A linearly-polarized and divergent light beam having a wavelength of 405nm is emitted from the semiconductor laser 1. This linearly-polarizedand divergent light beam is converted into a substantially parallel beamin the collimator lens 2, passes through the half mirror 4 and thedichroic prism 24, and the light path of the light beam is deflected by90 degrees in the deflection prism 5. Then, the light beam passesthrough non-sensitive regions of the polarized light selective aperturelimitation element 6 and the polarized light selective aberrationcorrection element 7, is incident on the object lens 8, and is condensedon the optical recording medium 9 a to be a fine spot. With this spot,data recording, reproduction, or erasure in the optical recording medium9 a is performed.

The light beam reflected from the optical recording medium 9 a isreflected by the half mirror 4. The reflected light beam is focused bythe detection lens 10, divided into plural divisional light beams in thelight beam divider 11 along different light paths, and arrives at thelight receiving element 12. The light receiving element 12 outputsinformation signals and servo signals.

Below, a description is made of operations of recording or reproducingdata in the optical recording medium 9 c, that is, the DVD opticalrecording medium C related to a wavelength of 660 nm, numerical apertureof NA=0.65, and a substrate thickness of 0.6 mm on the incidence side.

In recent years and continuing, in an optical pickup for DVD use, ahologram unit is widely used in which a light emission source and alight receiving element are provided in the same can, and a hologram isused to separate a light beam. The hologram unit 21 is an integration ofa semiconductor laser chip 21 a, a hologram 21 b, and a light receivingelement 21 c.

A light beam having a wavelength of 660 nm is emitted from thesemiconductor laser 21 a in the hologram unit 21, and this light beam isconverted into a substantially parallel beam by the collimator lens 22.The dichroic prism 24 has a characteristic of allowing a light beam inthe blue wavelength region to pass through, and reflecting a light beamin the red wavelength region. Hence, the incident light beam isdeflected to the deflection prism 5 by the dichroic prism 24, and thelight path of the light beam is deflected by 90 degrees in thedeflection prism 5.

Then, the numerical aperture of the light beam is limited to be NA=0.65in the polarized light selective aperture limitation element 6, and isdiffracted to a divergent light beam in the polarized light selectiveaberration correction element 7. Then, this light beam is incident onthe object lens 8, and is condensed on the optical recording medium 9 cto be a fine spot. With this spot, data recording, reproduction, orerasure in the optical recording medium 9 c is performed.

The light beam reflected from the optical recording medium 9 c isreflected by the deflection prism 5 and by the dichroic prism 24, and isfocused by collimator lens 22. By the hologram 21 b, the light beam isdiffracted to the light receiving element 21 c in the same can with thesemiconductor laser 21 a, and is detected by the light receiving element21 c. The light receiving element 21 c outputs information signals andservo signals.

In the present embodiment, the same as the blue-light optical recordingmedia A, B described in the preceding embodiments, the optical pickup isa compatible device which is able to record, reproduce or erase data inall of the blue-light optical recording medium A, the DVD opticalrecording medium C, and the CD optical recording medium C.

FIG. 12A is a perspective view of an aberration correction element usinga polarized light phase shifter according to the present embodiment, andFIG. 12B is a cross-sectional view of the aberration correction elementalong the line AA′ in FIG. 12A.

The aberration correction element of the present embodiment is notlimited to those utilizing diffraction as described in the firstembodiment, but can also employ a correction method using zero-orderlight.

Below, a phase shifter utilizing polarized light selectivity by asub-wavelength grating is taken as an example.

Further, the optical pickup in the present embodiment has the sameconfiguration as that for recording, reproducing or erasing data in boththe blue-light optical recording medium A and the blue-light opticalrecording medium B as illustrated in FIG. 1, and a phase shifter 7′ isused as the aberration correction element.

Below, a description is made of the aberration correction element whichutilizes the function of polarized light selective phase shifter.

When recording or reproducing data in the blue-light optical recordingmedium A, a light beam of a vertical polarization direction is incidentand passes through the phase shifter 7′, and when recording orreproducing data in the blue-light optical recording medium B, a lightbeam of a horizontal polarization direction is incident and passesthrough the phase shifter 7′.

As illustrated in FIG. 12A and 12B, the phase shifter 7′ includesconcentric planar patterns 7 e (first pattern) to 7 i (fifth pattern)with the optical axis as a center, and each of the concentric planarpatterns 7 e to 7 i has a step-like cross-section.

In each of the second pattern 7 f to the fifth pattern 7 i, asub-wavelength diffraction grating is formed with fine pitches. If aphase difference between the light passing through the gratings of finepitches and the light passing through regions other than the gratings offine pitches is an integral multiple of 2π, the incident light is notdiffracted and all of the incident light is transmitted through thephase shifter 7′. If the phase difference is not an integral multiple of2π, there is no light directly passing through the phase shifter 7′, andall components of the incident light are delayed corresponding to thephase differences, respectively, by the step-like portion of thegrating. If the shape of the steps of the gratings causing the delay isset to be opposite to the polarity of the generated aberration to becorrected, it is possible to correct the generated aberration.

Below, a description is made of operations of the phase shifter whenperforming recording or reproduction with the blue-light opticalrecording medium B.

Assume that the height of the steps of the sub-wavelength grating isuniform, and indicated by d, the refractive index of the phase shifteris represented by n1, and the refractive index in the horizontalpolarization direction of the incident light beam is represented by np,then, the light path length difference ΔL between every two neighboringregions is expressed by the following formula (13).ΔL=(np−n 1)d  (13)

In other words, with λ representing a wavelength, the phase difference δcan be expressed by the following formula (14). $\begin{matrix}{\delta = {( {{np} - {n1}} )\frac{2\pi\quad d}{\lambda}}} & (14)\end{matrix}$

FIG. 13A shows the wave front aberration to be corrected, and FIG. 13Bshows the wave front aberration after correction.

In FIG. 13A, the solid line in the upper portion indicates the wavefront aberration generated when condensing light on the blue-lightoptical recording medium B.

If a step-like phase is generated, as indicated by the dashed line inthe lower portion, and is added to the incident beam from the lightsource to the object lens, it is possible to cancel out the wave frontaberration caused by delay of the wave front in the light beam passingthrough the phase shifter, which is for correcting the aberration.

FIG. 13B shows a sum of the solid line (wave front aberration) and thedashed line (wave front delay due to the phase shifter) in FIG. 13A,that is, the wave front aberration after correction. As shown in FIG.13B, the wave front aberration after correction is much smaller than theoriginal aberration.

On the other hand, when recording or reproducing data in the blue-lightoptical recording medium A, because the phase shifter does not work, andthe light beam passes through the phase shifter without diffraction, thephase difference δ generated in the phase shifter is an integralmultiple of 2π, as expressed by the following formula (15).$\begin{matrix}{\delta = {{( {{ns} - {n1}} )\frac{2\pi\quad d}{\lambda}} = {2m\quad\pi}}} & (15)\end{matrix}$

In the present embodiment, the phase shifter and the aforesaid aperturelimitation element described in the first embodiment may be integratedtogether. For example, as illustrated in FIG. 12A, concentric planarpatterns 7 e (first pattern) to 7 i (fifth pattern) with the opticalaxis as a center constitute a phase shifter, and an aperture limitationelement may be provided in the peripheral region thereof; thereby, thephase shifter and the aperture limitation element are formed on the sameplane.

Third Embodiment

FIG. 14 is a schematic view illustrating an optical pickup according toa third embodiment of the present invention.

The optical pickup illustrated in FIG. 14 is a multi-beam optical pickuphaving a light source array including two channels. Below, a descriptionis made of a sub-wavelength grating functioning as a wave plate, with apolarization direction switching element in the multi-beam opticalpickup as an example.

A principal portion of the multi-beam optical pickup illustrated in FIG.14 includes light source unit 31, a collimator lens 2, a two-beam lightpath combination element 33, a half mirror 4, a deflection prism 5, anobject lens 8, a detection lens 10, a light beam deflection element 11′,and a light receiving element 12.

With the multi-beam optical pickup, it is possible to increase operatingspeed by recording or reproducing two tracks at the same time.Alternatively, with one light beam to perform recording, and with theother light beam to perform verification, it is possible to maintainhigh reliability. Further, with one light beam to perform recording,reproduction or erasure on a blue-light optical recording medium A, andwith the other light beam to perform recording, reproduction or erasureon a DVD optical recording medium or a CD optical recording medium, orwith one light beam to perform recording, reproduction or erasure on aDVD optical recording medium, and with the other light beam to operaterecording, reproduction or erasure on a CD optical recording medium, itis possible to make the optical pickup compact.

The light source unit 31 includes a semiconductor laser chip, and thesemiconductor laser chip includes two light emission sources, forexample, two light emission diodes, denoted as LD1 a and LD1 b. At thebackward stage of the semiconductor laser chip, a polarization directionswitching element 32 is arranged, the light beam from the LD1a passesthrough the sub-wavelength grating acting as a ½ wave plate, and thepolarization direction thereof is rotated by 90 degrees, beingperpendicular to the light beam from the LD2 a and passing through thepolarization direction switching element 32.

The light beam emitted from the light source unit 31 is converted into asubstantially parallel beam in the collimator lens 2. Here, theincidence angles of the two light beams passing through the collimatorlens 2 differ from each other. By using the two-beam light pathcombination element 33 described below for reducing the incidenceangles, the deviations of the light beams from the optical axis of theobject lens 8 can be reduced after the light beams are emitted from thecollimator lens 2. Afterward, the light beams pass through the halfmirror 4, and the light paths of the light beams are deflected by 90degrees in the deflection prism 5. Then, the light beam is condensed onthe optical recording medium 9 by the object lens 8.

The light beam reflected from the optical recording medium 9 apropagates back along the incidence light path, that is, it is reflectedby the half mirror 4, and arrives at a light beam deflection element11′. The light beam deflection element 11′ transmits or reflects ordiffracts the light beam reflected from the optical recording medium 9 aaccording to the polarization rotation angle, and directs all lightbeams to the same light receiving element 12. The light receivingelement 12 is appropriately divided into plural divisions according tothe methods of generating servo signals.

Based on the light beam reflected from the optical recording medium 9 a,the light receiving element 12 outputs tracking signals, focus signals,and reproduction signals to a subsequent later stage circuit (notillustrated).

Generally, if the light beam incident on the object lens 8 is inclinedrelative to the optical axis of the object lens 8, the quality of thespot formed on the disk surface cannot be maintained due to wave frontdegradation. Especially, the degradation increases when NA of the objectlens 8 a large.

FIG. 15 shows calculation results of a relation between an incidenceangle relative to the object lens and the wave front aberration.

In the present embodiment, the two-beam light path combination element33 is provided between the collimator lens 2 and the object lens 8 toreduce angular differences of the light beams relative to the opticalaxis of the object lens 8, the light beams being from the collimatorlens 2 and emitted from the laser diodes LD1 a and LD1 b.

It should be noted that although it is possible to reduce the angulardifference of two light beams by shortening the interval between twolaser diodes, because of difficulties in fabrication and thermalinterference between the two laser diodes, generally, the intervalbetween LD1 a and LD1 b is set to be a few tens μm.

According to the multi-beam optical pickup of the present embodiment,because of the polarization direction switching element 32, thepolarization planes of the two light beams emitted from the light sourceunit 31 can be set to be perpendicular to each other; hence, elementscapable of polarized-light selection can be used as the two-beam lightpath combination element 33.

FIG. 16A through FIG. 16C illustrate examples of the two-beam light pathcombination element 33.

As illustrated in FIG. 16A, the two-beam light path combination element33 may also be formed from a polarization hologram, which is used as apolarization selection unit, or from a Wollaston prism, as illustratedin FIG. 16B. Further, a beam shaping prism as illustrated in FIG. 16C,which is used for converting an elliptical beam into a nearly circularbeam, may also be as the two-beam light path combination element 33.

In addition, according to the multi-beam optical pickup of the presentembodiment, because of the polarization direction switching element 32,the polarization planes of the two light beams emitted from the lightsource unit 31 can be set to be perpendicular to each other; hence, inthese light beams, among the light beams reflected from the opticalrecording medium 9, the light beam for recording and the light beam forreproduction can be directed to the same light receiving element 12 bythe light beam deflection element 11′, according to the difference ofthe polarization planes. Therefore, it is not necessary to providerespective light receiving elements for recording and reproduction.

FIG. 17A and FIG. 17B illustrate examples of the light beam deflectionelement 11′.

As illustrated in FIG. 17A, the light beam deflection element 11′ may beformed from a polarization hologram, or from a Wollaston prism asillustrated in FIG. 17B.

FIG. 18A is a perspective view of a polarization direction switchingelement using functions of a wave plate given by a sub-wavelengthgrating as described above, according to the present embodiment, andFIG. 18B is a cross-sectional view of the polarization directionswitching element along the line AA′ in FIG. 18A.

In a wave plate using the sub-wavelength grating as illustrated in FIG.18A, the sub-wavelength grating produces anisotropy, and the phasedifference δ corresponding to the anisotropy can be expressed by thefollowing formula (16), $\begin{matrix}{\delta = {\frac{2\pi\quad{d1}}{\lambda}( {{n\quad 1} - 1} )}} & (16)\end{matrix}$

where λ represents the wavelength of the incident light, n1 represents arefractive index of the medium, and d1 represents a height of thegrating.

By appropriately selecting the refractive index of the medium n1, andthe height of the grating d1, this phase difference can be set to be πor π/2 thereby realizing various kinds of wave plates.

Below, a description is made of the polarization direction switchingelement using functions of a wave plate given by a sub-wavelengthgrating.

As illustrated in FIG. 18A, the surface of the polarization directionswitching element 32 is equally divided into two parts, and in one ofthe two parts, a sub-wavelength grating is formed, and in the other oneof the two parts, a sub-wavelength grating is not formed, that is, it ismerely a plate.

The light beam from LDa of the light source unit 31 passes through thesub-wavelength grating and the light beam from LD1 b of the light sourceunit 31 passes through the region without the sub-wavelength grating.

In order that the light beam from LD1 a of the light source unit 31passes through the sub-wavelength grating without diffraction, it issufficient to set the phase difference in the sub-wavelength grating tobe an integral multiple of 2π.

In addition, in order that the aberration is not generated, it ispreferable that the light path lengths from the light sources LD1 a andLD1 b to the collimator lens 2 be approximately the same. If the lightpath length from LD1 a to the collimator lens 2 is different from thelight path length from LD1 b to the collimator lens 2, this differenceinduces an aberration related to a defocus error of the collimator lens2.

The light path length L1 of the light beam passing through thesub-wavelength grating can be expressed by the following formula (17).$\begin{matrix}{{L\quad 1} = {( {\frac{{ns} + {np}}{2} + 1} )\frac{d1}{2}}} & (17)\end{matrix}$

It is sufficient to set the path length L1 to be the same as the lightpath length L2 of the light beam from the light source LD1 b and passingthrough the region where the sub-wavelength grating is not formed.

The light path length L2 is given by the following formula (18).L2=n1 d2  (18)

Fourth Embodiment

FIG. 19 is a schematic view illustrating an optical pickup according toa fourth embodiment of the present invention.

A principal portion of the optical pickup illustrated in FIG. 19includes a semiconductor laser 1 acting as a light source unit, acollimator lens 2, a half mirror 4, a deflection prism 5, a super highresolution element 35, an object lens 8, a detection lens 10, a lightbeam divider 11, and a light receiving element 12.

In the present embodiment, different from the third embodiment, the waveplate function may be provided in any portion of the light path. Forexample, a super high resolution element may be used. As is well known,the super high resolution element enables high density by reducing thesize of the light beam.

In the optical pickup as illustrated in FIG. 19, a linearly-polarizedand divergent light beam is emitted from the semiconductor laser 1, andthis linearly-polarized and divergent light beam is converted into asubstantially parallel beam in the collimator lens 2. Then the lightbeam passes through the half mirror 4, and the light path of the lightbeam is deflected by 90 degrees in the deflection prism 5. Then, aportion of the light beam near the optical axis is shielded by the superhigh resolution element 35, the laser beam without the portion near theoptical axis is incident on the object lens 8, and is condensed on theoptical recording medium 9 to be a fine spot.

Because of the super high resolution effect originating from lightshielding of the portion of the light beam near the optical axis, thelight spot has a much smaller size compared to the case when the superhigh resolution element 35 is not provided.

The light beam reflected from the optical recording medium 9 isconverted into a substantially parallel light beam again, and isreflected by the half mirror 4. The reflected light beam is focused bythe detection lens 10, divided into plural divisional light beams in thelight beam divider 11 along different light paths, and arrives at thelight receiving element 12. The light receiving element 12 outputsinformation signals and servo signals.

FIG. 20A is a perspective view of the super high resolution element 35functioning as a polarization direction switching element according tothe present embodiment, and FIG. 20B is a cross-sectional view of thesuper high resolution element 35 along the line AA′ in FIG. 20A.

As illustrated in FIG. 20A, a sub-wavelength grating is formed in aregion near the optical axis, and a not-illustrated polarization filteris uniformly formed on the opposite side to the sub-wavelength grating.As in the third embodiment, the sub-wavelength grating functions as a ½wave plate, which rotates the polarization direction of the light beampassing through the sub-wavelength grating by 90 degrees. Thepolarization filter is arranged to shield the light beam passing throughthe sub-wavelength grating, that is, the light beam whose polarizationdirection is rotated by 90 degrees in the sub-wavelength grating.Because the sub-wavelength grating is provided near the optical axis,the polarization filter is able to shield only the portion of the lightbeam near the optical axis.

The light-shielding area is not limited to a circular shape, but may beother shapes.

FIG. 21 is a perspective view of the super high resolution element 35illustrating an example of the light-shielding area.

As illustrated in FIG. 21, the light-shielding area may include multiplerectangular slices.

FIG. 22 is a perspective view of the super high resolution element 35illustrating another example of the light-shielding area.

As illustrated in FIG. 22, the light-shielding area may also be of across shape.

As mentioned in previous embodiments, because the phase differencebetween the light-shielding region and the non-light-shielding regionmay cause undesired aberration, it is preferable to adopt a structurewithout the phase difference, as in the third embodiment. Namely, thelight path length L1 of a light beam through the sub-wavelength gratingcan be expressed by the following formula (19). $\begin{matrix}{{L\quad 1} = {( {\frac{{ns} + {np}}{2} + 1} )\frac{d1}{2}}} & (19)\end{matrix}$

It is preferable that the light path length L1 be the same as the lightpath length L2 of the light beam passing through the region without thesub-wavelength grating. The light path length L2 is given by thefollowing formula (20).L2=n1 d2  (20)

Fifth Embodiment

FIG. 23 is a perspective view schematically illustrating an opticalinformation processing device according to a fifth embodiment of thepresent invention.

The optical information processing device 50 illustrated in FIG. 23 usesan optical pickup 53 to record, reproduce or erase data in an opticalrecording medium 55.

In the present embodiment, the optical recording medium 55 is a disk,and held in a carriage 56 acting as a protecting case. The opticalrecording medium 55, while being held in the carriage 56, is insertedinto the optical information processing device 50 through an entrance 51along an arrow “disk insertion”. Then, the optical recording medium 55is driven to rotate by a spindle motor 52, and the optical pickup 53executes recording, reproduction or erasure of data in an opticalrecording medium 55. Of course, it is not always necessary to put theoptical recording medium 55 in the carriage 56, and a bare opticalrecording medium 55 may be directly inserted into the opticalinformation processing device 50.

The optical pickup 53 in the present embodiment may be any one of theoptical pickups described in the previous embodiments.

While the present invention is described with reference to specificembodiments chosen for purpose of illustration, it should be apparentthat the invention is not limited to these embodiments, but numerousmodifications could be made thereto by those skilled in the art withoutdeparting from the basic concept and scope of the invention.

According to the present invention, in an optical device havingpolarized light diffraction functions realized by a sub-wavelengthgrating, and an optical pickup having such an optical device, a phasedifference does not occur between a region where the sub-wavelengthgrating is formed and a region where the sub-wavelength grating is notformed, and due to this, undesirable wave front degradation does nothappen. Therefore, it is possible to obtain an optical device havingfunctions of aperture limitation for polarized light selection,aberration correction, phase shifter, or functions of a wave plate, andby incorporating these optical devices, it is possible to obtain anoptical pickup of high compatibility, a multi-beam optical pickup, asuper-high resolution optical pickup, and an optical informationprocessing device using these optical pickups.

In addition, it is possible to obtain an optical device that can befabricated without a surface coating process, enables plural parts to beshared, and enables reduction of the size and the cost.

This patent application is based on Japanese Priority PatentApplications No. 2004-060748 filed on Mar. 4, 2004 and No. 2004-309361filed on Oct. 25, 2004, the entire contents of which are herebyincorporated by reference.

1. An optical device, comprising: a sub-wavelength grating formed in atleast one of a plurality of regions, a refractive index, a gratingheight or a duty ratio of said sub-wavelength grating being selectedsuch that a light path of an incident first light beam in thesub-wavelength grating is the same as a light path of the first lightbeam in neighboring regions, said first light beam being polarized andhaving a predetermined polarization direction.
 2. The optical device asclaimed in claim 1, wherein the sub-wavelength grating is superposed ona portion of a step-like surface, said surface having a plurality ofsteps, said portion including at least two steps.
 3. The optical deviceas claimed in claim 2, wherein the sub-wavelength grating is superposedon a grating having a period longer than a wavelength of the first lightbeam.
 4. The optical device as claimed in claim 3, wherein thesub-wavelength grating is formed in a circular-belt-like region todiffract an incident second light beam, said second light beam beingpolarized and having a polarization direction perpendicular to thepolarization direction of the first light beam.
 5. The optical device asclaimed in claim 3, wherein the sub-wavelength grating is formed in aninner portion of a circular-belt-like region to diffract an incidentsecond light beam, said second light beam being polarized and having apolarization direction perpendicular to the polarization direction ofthe first light beam.
 6. The optical device as claimed in claim 3,wherein the sub-wavelength grating is formed in a circular-belt-likeregion and in an inner portion of the circular-belt-like region toenable portions of the sub-wavelength grating in the circular-belt-likeregion and the inner portion of the circular-belt-like regionrespectively to diffract an incident second light beam to two differentdirections, said second light beam being polarized and having apolarization direction perpendicular to the polarization direction ofthe first light beam.
 7. The optical device as claimed in claim 2,wherein the sub-wavelength grating is formed in a plurality ofconcentric regions, and an incident second light beam has light paths ofdifferent lengths at different borders between each adjacent two of theconcentric regions, said second light beam being polarized and having apolarization direction perpendicular to the polarization direction ofthe first light beam.
 8. The optical device as claimed in claim 7,wherein the sub-wavelength grating is formed to have a plurality ofconcentric portions in an inner portion of a circular-belt-like regioncapable of diffraction.
 9. The optical device as claimed in claim 1,wherein the sub-wavelength grating functions as an m/2 (m is an integer)wave plate.
 10. The optical device as claimed in claim 9, wherein thesub-wavelength grating is formed in one of two regions obtained bysymmetrically dividing a surface of the optical device, and thesub-wavelength grating functions as a ½ wave plate relative to anincident second light beam, said second light beam being polarized andhaving a polarization direction perpendicular to the polarizationdirection of the first light beam.
 11. The optical device as claimed inclaim 9, wherein the sub-wavelength grating is formed inside a circleincluding a center portion on a plane, or in a region having a pluralityof rectangular slices, and the sub-wavelength grating functions as a½wave plate relative to an incident second light beam, said second lightbeam being polarized and having a polarization direction perpendicularto the polarization direction of the first light beam.
 12. A method ofproducing an optical device having a sub-wavelength grating formed in atleast one of a plurality of regions, wherein a light path of an incidentlight beam having a predetermined polarization direction in thesub-wavelength grating is the same as a light path of the incident lightbeam in neighboring regions, said method comprising the steps of:fabricating a mold having a fine grating shape on a surface thereof,said fine grating shape being determined by optical design; arranging alight-transmittive and thermal elastic resin on the surface of the moldand heating the resin to a melting temperature; pressing the heatedresin on the mold; cooling the resin gradually after the resin is shapedbased on the mold for shape transcription; and releasing the resin fromthe mold.
 13. An optical product including an optical device having asub-wavelength grating, wherein said sub-wavelength grating is formed inat least one of a plurality of regions, and a light path of an incidentlight beam having a predetermined polarization direction in thesub-wavelength grating is the same as a light path of the incident lightbeam in neighboring regions, said optical device being produced by amethod comprising the steps of: fabricating a mold having a fine gratingshape on a surface thereof, said fine grating shape being determined byoptical design; arranging a light-transmittive and thermal elastic resinon the surface of the mold and heating the resin to a meltingtemperature; pressing the heated resin on the mold; cooling the resingradually after the resin is shaped following the mold for shapetranscription; and releasing the resin from the mold.
 14. An opticalpickup for recording, reproducing or erasing data in at least twooptical recording media having numerical apertures of NA1 and NA2(NA1>NA2), respectively, said optical pickup comprising: an opticaldevice having a sub-wavelength grating, said sub-wavelength gratingbeing formed in a circular-belt-like region, and diffracting an incidentsecond light beam of a polarization direction perpendicular to apredetermined polarization direction of a first light beam; wherein inorder to diffract a light beam which is incident only when recording,reproducing or erasing data in the optical recording medium having thenumerical aperture of NA2, the circular-belt-like region of the opticaldevice is made to have diffraction functions to condense or scatter thelight beam to a position different from a condensing point in an innerportion of the circular-belt-like region.
 15. An optical pickup forrecording, reproducing or erasing data in a first optical recordingmedium which uses light of a wavelength λ1, has a numerical aperture ofNA1, and has a substrate of a thickness t1, said optical pickupcomprising: an optical device having a sub-wavelength grating, saidsub-wavelength grating being formed in an inner portion of acircular-belt-like region, and diffracting an incident second light beamof a polarization direction perpendicular to a predeterminedpolarization direction of a first light beam; wherein in order todiffract a light beam which is incident only when recording, reproducingor erasing data in a second optical recording medium which uses light ofa wavelength λ2 (λ2≧λ1), has a numerical aperture of NA2 (NA2≧NA1), andhas a substrate of a thickness t2 (t2≧t1), the inner portion of thecircular-belt-like region of the optical device is made to havediffraction functions to add an aberration having an opposite polarityto an aberration occurring when the light beam is condensed.
 16. Anoptical pickup for recording, reproducing or erasing data in a firstoptical recording medium which uses light of a wavelength λ1, has anumerical aperture of NA1, and has a substrate of a thickness t1, saidoptical pickup comprising: an optical device having a sub-wavelengthgrating, said sub-wavelength grating being formed in a plurality ofconcentric regions, and an incident second light beam having differentlight path lengths at different borders between each adjacent two of theconcentric regions, said second light beam being polarized and having apolarization direction perpendicular to a predetermined polarizationdirection of a first light beam; wherein in order to generate differentlight path lengths of a light beam which is incident only whenrecording, reproducing or erasing data in a second optical recordingmedium which uses light of a wavelength λ2 (λ2≧λ1), has a numericalaperture of NA2 (NA2≧NA1), and has a substrate of a thickness t2(t2≧t1), the concentric region of the optical device is made to havephase-step functions to add an aberration having an opposite polarity toan aberration occurring when the light beam is condensed.
 17. An opticalpickup for condensing light beams from a two-channel array light sourceto record, reproduce or erase data in an optical recording medium, saidoptical pickup comprising: an optical device having a sub-wavelengthgrating, said sub-wavelength grating being formed in one of two regionsobtained by symmetrically dividing a plane, and functioning as a ½ waveplate relative to an incident second light beam having a polarizationdirection perpendicular to a predetermined polarization direction of afirst light beam; wherein the two light beams output from thetwo-channel array light source are incident on the optical recordingmedium with the polarization directions of the two light beams beingsubstantially perpendicular to each other.
 18. An optical pickup,comprising: an optical device having a sub-wavelength grating, saidsub-wavelength grating being formed inside a circle including a centerportion on a plane, or in a region having a plurality of rectangularslices, said sub-wavelength grating functioning as a ½ wave platerelative to a second light beam having a polarization directionperpendicular to a polarization direction of a first light beam; and apolarization filter arranged between the optical device and an objectlens; wherein the polarization direction of a light beam incident on theoptical device is set to be substantially perpendicular near an opticalaxis of the light beam, and a portion of a light beam incident on theobject lens is filtered by the polarization filter to form a super highresolution beam spot on an optical recording medium.
 19. An opticalinformation processing device for recording, reproducing or erasing datain an optical recording medium, said optical information processingdevice comprising: a sub-wavelength grating formed in at least one of aplurality of regions, a refractive index, a grating height or a dutyratio of said sub-wavelength grating being selected such that a lightpath of an incident first light beam having a predetermined polarizationdirection in the sub-wavelength grating is the same as a light path ofthe first incident light beam in neighboring regions.